How to Grow Begonias in Australia

Bedding begonia, or wax begonia (Begonia semperflorens-cultorum hybrids), is a compact plant of the begonia family, Begoniaceae. It is grown for months of flowers in beds and pots, as an annual in cool gardens and a perennial in warm ones. The step most growers get wrong is covering the seed, which needs light to germinate.

Three mistakes that matter

1. Burying the seed. Begonia seed needs light to germinate, so seed covered with mix usually never comes up. Press each pellet onto the surface of moist seed raising mix and leave it uncovered, as set out in Sowing Seeds.
2. Sowing late, cold or dry. Seedlings grow slowly, and plants take about 16 to 21 weeks from sowing to flower. Sow under cover in warmth, early in your zone's window, and keep the surface wet while the seed germinates.
3. Planting into soggy soil or hot, exposed sun. Wet soil rots the fleshy stems at the base, and hot, windy spots burn leaves and flowers. Plant into free-draining soil with afternoon shade, and let the top 2 to 3 cm dry between waterings.

When to plant

Begonia seed germinates best at about 21 to 26°C and not below 15°C. Start it in trays under cover, often on a seedling heat mat, 12 to 16 weeks before planting out. Plant out once frost has passed and nights stay mild. One sowing carries a season.

In the tropics, finish sowing before the wet season build-up. Where frost lingers, keep young plants under cover until the last frost passes, traditionally around Melbourne Cup weekend.

Site, soil and setup

Morning sun with afternoon shade, or the dappled shade under a tree, suits bedding begonias in most Australian gardens. Cool districts can give them full sun, and bronze-leaved Monza Mix takes more sun than green-leaved kinds. They want rich, free-draining soil at about pH 5.8 to 6.8.

The organisms behind damping off and stem rot survive in soil and old plant material. Do not replant begonias where plants rotted the season before.

Container growing

Shallow roots suit bedding begonias to pots, window boxes and hanging baskets. One plant fills a 15 cm pot, and a 30 cm pot of about 13.5 L takes four to six. Use fresh potting mix and pots with drainage holes. Potting mix can carry Legionella bacteria: dampen it first, wear a mask and gloves, and wash your hands afterwards.

• Watering: check pots daily in hot weather and water at the base when the top 2 to 3 cm of mix is dry.
• Feeding: from the first flowers, give fish and seaweed blend at 25 mL per 9 L every fortnight.
• Position: morning sun and afternoon shade; in frosty districts, under cover before the first frost.

Sowing Seeds

Start begonias in cells under cover, because the dust-fine seed is not sown direct. It comes pelleted, each speck coated in a clay that dissolves in water.

Starting under cover

• Fill cells with fine seed raising mix and water it well before sowing.
• Set one or two pellets on the surface of each cell and press them in gently. Do not cover them.
• Mist the surface, then cover the tray with clear plastic or a lid to hold the humidity.
• Keep the tray at 21 to 26°C in bright light, out of direct sun. Seedlings usually appear in 2 to 4 weeks.
• Keep the mix wet for the first 10 days, then moist but never soggy, watering from below.

Potting up

Seedlings grow slowly at first. Lift the cover once most are up, and pot them on at two or three sets of true leaves.

• Lift each seedling by a leaf, never the stem, with a teaspoon or a plant label.
• Replant at the same depth in fresh potting mix and water in with seaweed solution at half strength, 15 mL per 9 L.
• Grow on in bright light, shaded from hot sun, until the roots fill the pot.

Before planting out, harden the plants off (stand them outdoors in shade for an hour or two a day, then longer, over about two weeks).

• Space plants 20 to 25 cm apart, so air moves between them.
• Set each plant at the depth it grew in its pot and water it in well.
• Mulch lightly, keeping the mulch clear of the stems.

Monza Mix is a bedding begonia with dark bronze leaves and mixed flower colours. The other begonia groups are raised another way.

Care by growth stage

Bedding begonias are light feeders, and their fleshy stems hold water through dry spells. Moist but never soggy soil and dry leaves keep them flowering and free of rot.

If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched in at the label rate can help.

Deadheading

Bedding begonias need no routine pruning. Picking off spent flowers keeps plants flowering and removes the ageing petals where grey mould starts.

• Every week or two: pinch off faded flower clusters and any fallen petals lying on the leaves.
• Leggy plants: pinch off the top 2 to 3 cm of long stems, and the plant branches from below.
• Warm zones: cut back tired plants in late winter to early spring, just above a leaf joint.
• Frosty zones: trim potted plants in autumn and keep them under cover, watered sparingly, until spring.

Companion planting

Good partners share the begonias' part shade or stand behind them.

Grows well with

• Petunias: bedding annuals planted out at the same time, taking the sunnier end of the bed.
• Lobelia and sweet alyssum: low edging plants that suit the same front-of-bed position.
• Ferns and shade-tolerant shrubs: they cast the dappled light begonias like.

Keep apart

• Pumpkins: their runners and broad leaves smother low bedding plants.
• Corn and other tall, dense crops to the north: begonias flower less in deep shade.

Harvesting Blooms

The first flowers usually open 16 to 21 weeks from sowing. Plants flower until the cold stops them, or all year in warm zones.

• For a small posy, snip sprigs just above a leaf joint, and the plant branches below the cut.
• For new plants, cut a healthy non-flowering tip and stand it in water to root.
• Pot up rooted cuttings once the roots are 4 to 5 cm long.
• For seed, snip off the small pods behind faded flowers once brown and dry, and open them over white paper.

Keep saved seed in a paper envelope in a cool, dry place; seed from a mix can grow unlike its parents. In beds and pots the flowers also draw bees.

Common problems

Seed does not come up. See the first two points in Three mistakes that matter. Read more

Seedlings collapse at soil level. Damping off (a rot of seedlings at soil level, chiefly caused by Pythium species and Rhizoctonia solani) strikes in wet, stale mix. Read more

Seedlings grow tall and pale. They are stretching for more light. Read more

Pale brown or yellow leaf patches. This is sunburn, so move plants out of direct midday sun.

Yellow leaves and soft, rotting stems. Overwatering and poor drainage bring on root and stem rot; see the third point in Three mistakes that matter.

Few flowers. Plants in deep shade flower less, so give them brighter, dappled light.

Pests and diseases

Snails take seedlings, and mildew, mould and sap-suckers follow crowding, damp or dry heat.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They chew seedlings and soft leaves after dark. Hand-pick them by torchlight about two hours after sunset, set beer traps and band pots with copper. Read more

Powdery mildew (Erysiphe begoniicola). A white to grey powdery coating spreads on the leaves, often underneath, and badly affected leaves wither. Remove affected leaves and spray 1 part full-cream milk to 10 parts water. Read more

Grey mould (Botrytis cinerea). Spent petals, flowers and leaves turn soft and brown under grey mould in cool, damp weather. Remove affected parts promptly.

Two-spotted mite (Tetranychus urticae). These tiny sap-suckers leave pale, stippled leaves, and they build up fast in hot, dry weather. Hose the undersides of the leaves. Read more

Greenhouse whitefly (Trialeurodes vaporariorum). Tiny white insects rise in a cloud when the plant is brushed, leaving sticky honeydew. Hang yellow sticky traps and hose the leaf undersides. Read more

Prevention first

• Keep to the spacing in Sowing Seeds, so dew and rain dry off the leaves quickly.
• Water the soil, not the leaves, and do it early in the day.
• Use fresh seed raising mix and clean cells for every sowing.
• Walk the beds with a torch after dark while the plants are small.

Tips

1. Keep a thermometer in the propagating tray, because the mix, not the room, needs to sit at 21 to 26°C.
2. Begonias can be sensitive to insecticidal soap, so test any spray on a few leaves first.
3. All begonias are toxic to dogs and cats if chewed, and the roots most of all, so plant them where pets do not dig.